1. Field of the Invention
The present invention generally relates to globoid worm gears, and in particular, to a globoid worm gear speed reduction apparatus for improving the transfer of rotational power from a globoid worm gear to an engaged worm wheel.
2. Prior Art
The prior art discloses several forms of worm gearing in general, and globoid worm gears in particular. As an example of the prior art, Applicants refer to Japanese application Ser. No. 51-72808, filed June 22, 1976 and Japanese Patent No. 62-19970 issuing thereon. The corresponding United States application matured into U.S. Pat. No. 4,184,796 issued Jan. 22, 1980. One of the Applicants, M. Maki, is a co-inventor of the method claimed in U.S. Pat. No. 4,184,796 (the "'796 patent"). In the '796 patent, the method of forming globoid worm gears is disclosed in a manner shown in FIG. 3 of the drawing. The helical thread of the worm gear is cut through the use of a cutting tool employing a frusto-conical surface. In the method taught in the '796 patent, the center of rotation of the cutting tool, when swinging along the axis of the worm gear, is equal to the distance between the axes of rotation of an engaged worm wheel and worm gear, the speed of rotation of the cutting tool being substantially equal to the worm wheel.
The prior art fully discloses the structure of what is conventionally referred to as a Hindley globoid worm gear and the method of forming same. As disclosed in the prior art, conventional Hindley worm gears are generally formed through the use of tools employing linear surfaces. In the present invention, the linear tool is replaced with one having a frusto-conical surface, the cross-section thereof being substantially in the form of an isosceles triangle. The theory is fully described in the article entitled "An Investigation of Secondary Action on Skew Gears" by T. Sakai and M. Maki appearing in The Transactions of American Society of Mechanical Engineers, Journal of Engineering for Industry, published in February, 1974. One of the joint inventors of the present invention, M. Maki, is one of the authors.
The prior art discloses a number of improved methods for enlarging the engagement area of Hindley worms and gears. The present invention provides for increased engagement between the globoid worm gear and worm wheel and thereby increases the horsepower transmission characteristics. The present invention solves the problems inherent in those disclosed in the prior art by utilizing a globoid worm gear formed by a cutting tool employing a frusto-conical surface. To achieve this objective, in the formation of the worm gear, the cutting tool swings along an arc in a plane including the axis of the worm gear, the distance between the center of rotation of the cutting tool and the axis of the worm gear being incrementally increased to a distance which is greater than the distance between the axis of the worm wheel and the axis of the globoid worm gear when engaged. Under this condition, the speed reduction ratio existing at the time of the formation of the worm gear is incrementally greater than the speed reduction ratio calculated with respect to the rate of rotation of an engaged worm wheel and worm gear. The structure of a worm thread in accordance with the present invention is equivalent to the worm thread of a conventional Hindley globoid worm only at the center of the worm gear. The structure of the worm thread reflects its formation by a cutting tool employing a frusto-conical surface longitudinally moved along the worm about a center of rotation spaced incrementally beyond the center of an engaged worm wheel. The resulting worm gear provides for increased engagement with a worm wheel thereby increasing the horsepower transmission characteristics.